Field
The disclosed concept pertains generally to power distribution systems and, more particularly, to systems, such as microgrid systems, that detect unintentional islanding of a microgrid. The disclosed concept further pertains to methods of detecting islanding and, more particularly, to methods of detecting islanding for a microgrid system.
Background Information
In electric utility systems, a grid outage condition can cause the creation of an “unintentional island” including the electrical load(s) and the power generation source(s). Such an island is undesirable and is of a particular concern in distributed power generation systems having a number of power generation sources and loads coexisting on a distribution feeder. For example, such an island can result in an abnormal voltage or frequency being supplied to the load. Furthermore, through back-feeding, such an island can present a safety hazard to workers for upstream power circuits.
When a microgrid is electrically connected to the utility grid, it is necessary to match the microgrid frequency and voltage amplitude with that of the grid. The microgrid uses the grid as its reference and generates an output voltage that is synchronized with the grid. If the grid becomes electrically disconnected, then the microgrid does not see any change in frequency or voltage and will continue to supply power if the output power of the microgrid matches with the local load demand on the grid. Such a condition is known as islanding, which can have substantial safety and performance implications.
FIG. 1 shows a power distribution system 2 including an example utility grid 4 and a microgrid 6. Depending on the place of loss of electrical connection to a grid (e.g., without limitation, the utility grid 4 of FIG. 1; a non-utility grid (not shown)), in a microgrid, such as 6, the power output from a number of available microgrid power sources 8 (e.g., without limitation, a number of inverters and a corresponding number of inverter-based power sources; a diesel generator) may be equal to the power input by a number of available microgrid loads 10 within the microgrid 6 and a number of other system loads 12 outside of the microgrid 6, but still on the connected grid 4. When the power input by the number of system loads 12 and the number of microgrid loads 10 together equals the power output from the number of microgrid power sources 8 within the microgrid 6, the real and reactive power into the grid 4 can be zero (e.g., P=Q=0). Switch S provides a point 20 of loss of connection to the grid 4, which is a safety hazard. For example, when the switch S is opened and the microgrid isolation switch 18 is closed, an unintentional island 16 is formed. The real and reactive power through the upstream switch S is zero. Hence, there is the possibility that if a lineman opens the switch S, the point 20 of the switch S has a voltage. This is a safety hazard for the lineman.
The standard IEEE 1547 addresses the requirement for anti-islanding of distributed resources. However, the above problem is for anti-islanding of a microgrid or a number of power sources working closely within a microgrid.
There is room for improvement in microgrid systems.
There is also room for improvement in anti-islanding methods.